Wind turbines having blade pitch systems for adjusting the pitch angle of the blades have been known for many years, typically employing electric pitch drives or hydraulic pitch drives. Early examples of hydraulic pitch drives are disclosed, for example, in U.S. Pat. No. 4,348,155 and U.S. Pat. No. 4,352,634, both of United Technologies.
Emergency feathering systems for wind turbines with variable pitch of the blades have been known for many years for turning the blades to the feathered position, i.e., where the blade is oriented with the chord parallel to the wind and the leading edge pointed in the direction of the wind, so that damages to the wind turbine are avoided during high wind.
It is one aspect of the present invention to provide an improved system for controlling the emergency feathering system so that malfunctions of parts of the control system to a lesser extent will prevent the wind turbine from operating and producing power, and thus improve the availability of the wind turbine.